MAINTENANCE: Automated closing of pull requests.

This commit exists to close the following pull requests on Github:

Closes apache#2849 (close requested by 'srowen')
Closes apache#2786 (close requested by 'andrewor14')
Closes apache#4678 (close requested by 'JoshRosen')
Closes apache#5457 (close requested by 'andrewor14')
Closes apache#3346 (close requested by 'andrewor14')
Closes apache#6518 (close requested by 'andrewor14')
Closes apache#5403 (close requested by 'pwendell')
Closes apache#2110 (close requested by 'srowen')